In drilling a borehole to recover oil from the earth, it is often helpful to turn or steer the downhole drill bit toward or away from subterranean targets. To facilitate this geophysical steering, drillers need to know drill bit location on the surface seismic section. The location of targets ahead of the bit is also required, as well as some warning or indication of drilling hazards such as over-pressured formations or thin, shallow gas intervals. Surface seismic surveys generally include this information, but resolution and depth location is poor because surface seismic surveys are time based (rather than depth based). For example, to determine the depth of a reflection, a speed of sound for the formation must be known. Consequently, these systems require depth calibration to accurately determine locations of target horizons or drilling hazards. Traditionally, this calibration has been provided by either offset well sonic data or wireline checkshot data in the current well. Offset data is often inadequate however due to horizontal variations in stratigraphy between wells. Wireline checkshots require tripping (i.e., removing) the bit out of the hole and are often prohibitively expensive.
During surface seismic surveys, a plurality of seismic sources and seismic receivers are placed on the surface of the earth. The seismic sources are triggered in a predetermined sequence, resulting in the generation of seismic waves. These seismic waves travel downward through the earth until reflected off some underground object or change in rock formation. The reflected seismic waves then travel upward and are detected at the seismic receivers on the surface. One or more clocks at the surface measure the time from generation of the seismic waves at each source to the reception of the seismic waves at each receiver. This gives an indication of the depth of the detected object underground. However, the exact speed of sound for these seismic waves is unknown, and thus, the exact depth of the detected object is also unknown. To more closely measure the exact speed of sound, a "wireline checkshot" may be used to calibrate depth measurements. During a "wireline checkshot," a receiver on a "wireline" is lowered a known distance into an already-drilled borehole. A surface seismic source is then triggered and the time is measured for the seismic wave to travel to the wireline receiver. Because the depth of the wireline receiver is known, an average interval velocity indicating the average speed of the seismic wave can be determined with some degree of accuracy.
Attempts have been made to provide "reverse vertical seismic profiling" while drilling. This approach transposes the downhole location of the receiver with the surface location of the seismic source. These attempts have been based mainly on using the drill bit as a source of a seismic wave, with an array of suitable receivers spread on the surface. These methods, however, work with only specific bit types, and they will not work in highly deviated wells. Also, attempts have been made to generate sources downhole that permit reverse vertical seismic profiling. A source by Klaveness, U.S. Pat. No. 5,438,170 incorporated herein by reference, is akin to a drilling jar and generates large tool modes and tube waves and poor seismic signals. Another known source vibrates the bit with a piezoelectric. It has a very short range and can only be used with downhole receivers. Another drawback is that for formations directly ahead of the bit, downhole sources and receivers cannot distinguish between high-pressure formations and other reflective boundaries. An air gun has also been placed downhole, but requires air lines from the surface. Air guns and water guns are not ideal downhole sources because they are localized sources that create large tube waves, require intrusive pressure systems, and may damage the formation.